Hypersonic vehicles frequently require sharp-featured nose tips and wing leading edges to reduce drag. However, the geometry of these edges increases the convective heat flow to the surface, ultimately increasing the overall temperature of the component with temperatures as high as 2300 K or more. Materials under consideration for hypersonic structures have often focused on ultra-high temperature ceramics (UHTCs) such as ZrB2 and HfB2, with SiC added for oxidation resistance. Such materials are typically hot pressed in order to achieve densities in excess of about 90 percent, making them expensive to produce and limiting their possible as-pressed shapes to simple geometries.
Embodiments of the present novel technology limit heating of hypersonic structures by radiating heat away with high emissivity structures and/or coatings. By reducing heat flow to these components, it may be possible to reduce oxidation rates and/or retain the mechanical performance necessary for the success of UHTC structures.
ZrB2-based UHTCs are examples of suitable high emissivity coatings that can radiate heat away from hypersonic leading edges and nose cones. Electronic and atomic processes are thought to lead to the high emissivity of these oxides, and additional materials may be identified by understanding these processes.
Materials characterization and total hemispherical emissivity testing may also be used to explain and potentially improve the performance and/or identification of the high emissivity materials.
Insight into the fundamental science governing favorable emission bands from rare-earth oxides may be attained using modeling to characterize the optical and IR properties of candidate materials via density functional theory (DFT) calculations. The use of atomistic predictive capabilities can guide the design of high ϵ coatings. Suspension plasma spray is a helpful processing approach for high ϵ coatings, affording a means to tailor the composition of the coatings. Rare-earth oxides that possess a high emissivity can be used as high ϵ surface in hypersonic environments to re-radiate the heat back to the environment, limiting the amount of heat absorbed by the underlying ceramic structure.
Hypersonic vehicles, including missiles and manned aircraft frequently use surfaces with sharp features. These surfaces include nose tips and wing leading edges, with the sharp geometry reducing the drag on the vehicle. However, the geometry of these edges increases the convective heat flow to the surface, ultimately increasing the overall temperature of the component with temperatures reaching as high as 2273 K in the stagnation region of a sharp leading edge. Prior attempts to mitigate the impact of these high temperatures have often focused on diboride ceramics, including ZrB2 and HfB2, with SiC added so that a silica scale is formed during service that provides protection from further oxidation. For example, by adding about 20 volume percent SiC to ZrB2, silica will form on the surface after oxidation at about 1473 K. The silica scale remains protective for the underlying structure up to 1773 K before it begins to evaporate.
As the hypersonic environment may typically feature temperatures of about 500 degrees higher than the evaporation threshold temperature of silica, there remains a need for increasing the oxidation resistance of ZrB2 and other high-temperature ceramic materials at extreme temperatures. The present novel technology addresses this need.